Aluminum flake pigment is widely known for its ability to impart metallic luster to coating compositions in which it is used. Although aluminum flake is used in many different decorative applications, the metallic luster characteristic of this pigment is particularly desirable in automotive finishes. While the aluminum flake pigments have proven readily useful in coating compositions based on organic solvent systems, difficulties have been encountered in attempting to use the same pigments in aqueous solvent systems, e.g., solvent systems containing about 80 percent water. It will be understood that aqueous coating systems are becoming more and more attractive as a means of eliminating the problems of pollution control and safety which are inherent with the organic solvent systems.
In aqueous medium, aluminum flake pigment can undergo reaction with water with accompanying evolution of hydrogen gas. This phenomenon, known as "gassing," can be especially troublesome when the pigment is stored in aqueous medium in sealed containers. Gassing can be lessened, at least to some extent, by encapsulating the aluminum flakes with dense amorphous silica. However, since the silica coating is conventionally applied to aluminum flake in aqueous suspension, e.g., as per Iler U.S. Pat. No. 2,885,366, gassing can occur even during the preparation or isolation of encapsulated flake. In any event, the quantity of silica usually necessary to impart even partial resistance to gassing, can dull the aluminum flake to such an extent as to render it undesirable for finishes requiring metallic luster.